Contributing to libjpeg-turbo

libjpeg-turbo is a stable and mature product with a worldwide reach, it is an ISO/ITU-T reference implementation of the JPEG standard, and its maintainer does not earn a salary for maintaining it. Thus, not every code contribution can or will be accepted into the libjpeg-turbo code base. In order to maximize the chances that a code contribution is acceptable, please adhere to the following guidelines:

1. If the code contribution is a bug fix, then please ensure that enough information is provided so that the maintainer can readily reproduce and document the bug. That information should include:

   • A clear and concise description of the bug
   • The steps and (if applicable) images necessary to reproduce the bug
   • The compilers, operating systems, and CPUs with which the bug was observed
   • The versions of libjpeg-turbo with which the bug was observed
   • If the bug is a regression, the specific commit that introduced the bug (use git bisect to determine this)

2. If the code contribution will implement a major new feature, then please contact the project maintainer (through a GitHub issue, direct e-mail, or the libjpeg-turbo-devel mailing list) prior to implementing the feature. In general, major new features that are potentially disruptive to the quality of libjpeg-turbo are unlikely to be accepted unless:

   • The new feature is within the existing scope of libjpeg-turbo.
   • The new feature has been thoroughly regression tested and benchmarked on all of the supported platforms that are potentially affected by it.
   • The new feature has been documented clearly and concisely in the change log and (if applicable) the libjpeg and TurboJPEG API documentation and man pages.
   • The code implementing the new feature is formatted consistently with the rest of the libjpeg-turbo code base (use checkstyle to validate this.)
   • The new feature does not introduce new members into the exposed libjpeg API structures (doing so would break backward ABI compatibility.)
   • The new feature does not alter existing libjpeg-turbo usage or development workflows.
   • The code implementing the new feature is elegant, easily maintainable, and causes the least possible amount of disruption to the libjpeg-turbo code base.
   • The new feature is based on the dev branch of the libjpeg-turbo repository.

   ... OR ...

   • Your organization is prepared to pay for the labor necessary to ensure all of the above. Even the most well-written patches can still require a significant amount of labor to clean up, test, and integrate. (See above RE: the maintainer not earning a salary.)

   Specific types of features that will not be accepted:

   • Features that extend the scope of libjpeg-turbo to support image formats other than DCT-based JPEG and JFIF
   • Features that extend the scope of libjpeg-turbo to support image processing algorithms that are not necessary for JPEG compression or decompression
   • Extensions to the JPEG format that have not been approved by the appropriate standards bodies
   • Non-trivial performance enhancements that have less than a 5% overall impact on performance

3. If the code contribution is a build system enhancement, then please be prepared to justify the enhancement. In general, build system enhancements are unlikely to be accepted unless:

   • The enhancement is potentially beneficial to a significant number of upstream libjpeg-turbo users/developers. (If the enhancement is only beneficial to a downstream project, then it does not belong here.)
   • The enhancement has been tested with the following CMake versions:
     • The earliest version of CMake that libjpeg-turbo currently supports (refer to CMakeLists.txt)
     • The most recent major version of CMake
     • (if applicable) The earliest version of CMake with which the enhancement can be used
   • The enhancement has been tested on all of the major platforms (Mac, Linux, Windows/Visual C++, Windows/MinGW) that are potentially affected by it.
   • The enhancement does not introduce new build system requirements or CMake variables unless absolutely necessary.
   • The enhancement does not alter existing libjpeg-turbo development workflows.

   Specific types of build system enhancements that will not be accepted:

   • Enhancements that allow libjpeg-turbo to be built from a subdirectory of a downstream repository. These enhancements are not maintainable in the upstream libjpeg-turbo build system. Use the CMake ExternalProject_Add() function instead.
   • Enhancements that introduce new (non-CMake-based) build systems